1. Field of the Invention:
This invention is concerned with the preparation of large single crystals of alkaline earth halides and more particularly to the preparation of alkaline earth chloride single crystals.
2. Description of the Prior Art:
Alkaline earth chlorides are reported to be grown from the melt in a vacuum or inert atmosphere. The resulting crystals are highly hydrolyzed; i.e., they contain an OH.sup.- mole fraction .gtoreq.10.sup.-3 and exhibit a poor environmental stability which ranges from a hygroscopic behavior, as in the case of BaCl.sub.2 and SrCl.sub.2, to deliquesence, as in the case of CaCl.sub.2.
These crystals are useful in the laser technology fields as laser window materials. They have utility in other optical applications requiring high optical transmission efficiencies and long term stability. However, their utility is limited by the afore-mentioned characteristics which we attribute to the presence of anion and cation impurities in the crystals in general and in particular to the presence of OH.sup.31 impurities.
Stockbarger sought to remove O.sup.-2 from CaF.sub.2 melts by using PbF.sub.2 as a scavenger in accordance with the following equation (D. C. Stockbarger, J. Opt. Soc. Am. 39, 731, 1949) EQU PbF.sub.2 (c)+O.sup.-2 (c).revreaction.2F.sup.- (c)+PbO(c) (1)
However, the process was defective in that excess PbF.sub.2 and PbO was volatilized, residual amounts of the scavenger throttled the growth rate, and rehydrolysis of the CaF.sub.2 melt occurred.
Applicants know of no prior art crystal growth processes which yield alkaline earth chloride crystals exhibiting ultra-low optical absorption characteristics and longer term environmental stability in a consistent manner that is suitable for large scale production operations.